Solid Oxide Fuel Cell Interconnect Research Articles

Influence of Cr Content on the Mechanical, Oxidation, and Electrical Properties of Powder Metallurgy Ferritic Stainless Steel for Solid Oxide Fuel Cell Interconnect

Influence of Cr Content on the Mechanical, Oxidation, and Electrical Properties of Powder Metallurgy Ferritic Stainless Steel for Solid Oxide Fuel Cell Interconnect

Design and conductivity optimization of La0.3Sr0.7TiO3/La0.8Sr0.2MnO3 bi-layer structures as tubular segmented-in-series solid oxide fuel cell interconnect☆

This study focused on meeting the stringent stability requirements of tubular segmented-in-series solid oxide fuel cells (SOFCs) in reducing and oxidizing atmospheres. To address this challenge, a bi-layer perovskite ceramic interconnect was designed by controlling the oxygen partial pressure, because of the strong correlation between the conductivity of strontium-doped lanthanum titanate (LST) and the oxygen partial pressure. The LST powder was prepared using solid-phase and sol-gel methods, and their influence on particle size and sintering behavior was compared. LST/lanthanum strontium manganite (LSM) bi-layer ceramic interconnects with varying thicknesses were fabricated through screen printing and co-sintering. The results demonstrate favorable interfacial bonding and excellent chemical compatibility between the ceramic layers. The conductivity of the bi-layer interconnect exhibits a temperature-dependent behavior, peaking at 550 °C. Simulation calculations and research findings validate that the conductivity of the bi-layer interconnect is determined by the thickness of the LSM layer and the oxygen partial pressure at the interconnect interface. Optimal conductivity is achieved with a bi-layer interconnect consisting of approximately 15 μm of LST and 4 μm of LSM. This can be attributed to the efficient regulation of oxygen partial pressure at the interface, effectively mitigating LSM decomposition caused by low oxygen partial pressure and the subsequent reduction in conductivity. These results provide valuable fundamental data and methodology for the development of high-performance interconnects for tubular segmented-in-series SOFCs.

Unveiling niobium pentoxide and cerium oxide-dispersed ferritic stainless steel for solid oxide fuel cells interconnects

Herein, the effect of niobium pentoxide (Nb2O5) and nano-sized cerium oxide (CeO2) on oxidation behavior and area specific resistance (ASR) of the ferritic stainless steel has been investigated for the development of robust metallic interconnects. An oxide-dispersed ferritic stainless steel alloy is fabricated using the powder metallurgy (PM) process. The morphologies of the alloy before and after oxidation are observed through Scanning Electron Microscope (SEM), Scanning Transmission Electron Microscopy (STEM) and Secondary Ion Mass Spectrometry (SIMS) analyses. Addition of Nb2O5 in the alloy formed a C14 Laves phase of (Fe, Cr)2(Nb, Si). The Laves phase and CeO2 controlled the oxidation behavior of the ferritic stainless steels, consequently reducing the oxide scale thickness and improving the ASR. An alloy with 1 wt% Nb2O5 and 3 wt% CeO2 showed an ASR value of 27.1 mΩ cm2 at 800 °C for 1000 h, which is similar to the ASR value of the commercial Fe–Cr alloy. The results of this study indicate that Nb2O5 and CeO2-dispersed ferritic stainless steel using the PM process can be more advantageous than the conventional process for manufacturing metallic interconnects with complex shapes and reducing production costs.

Effect of the simultaneous addition of lanthanum and nickel on the oxidation behavior and related area-specific resistance of ferritic stainless steels for solid oxide fuel cell interconnects

The effects of simultaneous lanthanum and nickel addition on the oxidation behavior and area-specific resistance (ASR) of stainless steels for fuelcell interconnects were investigated. The La and Ni addition promoted the entire growth of the oxide layer and increased the roughness of the metal/oxide interface. Despite the thicker oxide scale, these alloys prevented the increase of ASR compared to the alloy with only Ni added. The void analysis indicated that simultaneous La and Ni addition could prevent void growth, leading to lower ASR. The above findings can be interpreted in terms of the fast La diffusion and convoluted metal/oxide interface.

Evaluation of selected Fe–Cr steels under single- and dual-atmosphere conditions for intermediate-temperature solid oxide fuel cell interconnect applications

For Intermediate-Temperature SOFC, the lower operating temperature allows for a broader range of steel grades to be used as interconnects. This work compares the oxidation performances of six Cr2O3-forming steels and one Al2O3-forming steel, under single- and dual-atmosphere conditions.Single-atmosphere exposures in combination with Cr(VI) evaporation measurements were carried out at 600 °C. Except for AISI 430 all the Cr2O3-forming steels showed protective behaviour with similar Cr(VI) evaporation values and low area specific resistance (ASR) values. Kanthal EF101 showed the lowest level of Cr(VI) vaporisation but substantially higher ASR values.Dual-atmosphere exposures were carried out at 600 °C in Ar-5%H2–3%H2O//air-3%H2O. All Cr2O3-forming steels suffered various degrees of breakaway corrosion. Therefore, operation under dual-atmosphere conditions is likely the life-time-limiting failure mode.

TiC and (Ti,Nb)3SiC2 based dual-layer coating on SUS430 for solid oxide fuel cell interconnects

TiC and crystalline (Ti,Nb)3SiC2 based dual-layer is prepared on SUS430 alloy by DC magnetron sputtering followed by annealing at 1000 °C for 20 h. The oxidation of TiC/TNSC coated alloy at 800 °C for 500 h follows the parabolic law with a low rate constant of 5.4 × 10−15 g2 cm−4 s−1. The formed oxide scale is composed of Cr2O3/MnCr2O4 dominated inner layer and outer layer of Nb-doped TiO2 and SiO2 uniform mixture. Owing to the barrier effect of TiC layer and dense oxide scale, outward diffusion of Cr and Mn is efficiently suppressed, resulting in the low Cr content in the outer oxide layer, which is beneficial to the reduced volatilization of Cr oxide. Furthermore, the ASR of the formed oxide scale at 800 °C is 15.87 mΩ cm2, suggesting excellent conductivity. It is concluded that the TiC/(Ti,Nb)3SiC2 coating can significantly improve the oxidation resistannce and electrical property of SUS430 alloy, endowing it potential application as SOFC interconnect protective coating.

Applying the Protective Mn–Co–La2O3 Coating on Crofer 22 APU Ferritic Stainless Steel Used as Solid Oxide Fuel Cell Interconnects

Applying the Protective Mn–Co–La2O3 Coating on Crofer 22 APU Ferritic Stainless Steel Used as Solid Oxide Fuel Cell Interconnects

Investigation of surface hardness, thermostability, tribo-corrosion, and microstructural morphological properties of microwave-synthesized high entropy alloy FeCoNiMnCu coating claddings on steel

Deposition of high entropy alloy FeCoNiMnCu on SS-304 was carried out by microwave energy for application in “solid oxide fuel-cell (SOFC) interconnects”. The ball-milling has been performed by taking “Fe, Co, Ni, Mn, and Cu” in equal 20 wt. % of before deposited on SS-304 substrate. The deposited steel with 20% Fe 20% Co 20% Ni 20% Mn 20% Cu high entropy alloy (HEA) was exposed to thermal-exposure in the air for up to 10 weeks at 800 °C. The uniform cladding distribution of 20% Fe 20% Co 20% Ni 20% Mn 20% Cu HEA particles can be apparently observed on SS-304 substrate by utilizing Scanning Electron Microscope (SEM), and Optical microscopy analysis. Homogeneity in the interfacial layer was evident by employing Scanning Electron Microscope (SEM) characterization. Results have indicated that after the thermal exposure of deposited steel with 20% Fe 20% Co 20% Ni 20% Mn 20% Cu in the air for up to ten weeks at 800 °C, a “protective Cr2O3 layer”, and “high-entropy spinel coating” of (Fe, Co, Ni, Mn, Cu)3O4 have been formed. During microwave cladding, the emergence of harder-phases has contributed to the raised hardness. The wear behavior after coating of 20% Fe 20% Co 20% Ni 20% Mn 20% Cu HEA on SS-304 substrate has significantly enhanced due to the strengthened wear resistance and hardness of the coatings. Findings have exhibited that the formation of (Fe, Co, Ni, Mn, Cu)3O4 phase is a potential coating material for “SOFC interconnects” applications. Moreover, the cladding of SS304 with a composition of 20% Fe, 20% Co, 20% Ni, 20% Mn, and 20% Cu has demonstrated remarkable stability under thermal expansion studies. As the findings have revealed that the composite cladding has efficiently withstand significant variations in volume when subjected to elevated temperatures for a prolonged period of time, thus, exhibiting its superior thermal stability for SOFC-interconnect applications. Furthermore, the SEM images of the cladding surface, surface hardness, and tribocorrosion behavior of the coated material have been observed to identify the 20% Fe 20% Co 20% Ni 20% Mn 20% Cu HEA coating effect on SS-304 steel-substrate.

The Preparation and Properties of Ti(Nb)-Si-C Coating on the Pre-Oxidized Ferritic Stainless Steel for Solid Oxide Fuel Cell Interconnect.

Cr2O3 scale growth and volatilization are the main cause of the performance degradation of solid oxide fuel cells (SOFCs) with an Fe-based ferritic stainless steel (FSS) interconnect. In this work, an amorphous Ti(Nb)-Si-C coating is prepared on the pre-oxidized SUS430 with D.C. magnetron sputtering as the protective coating. The amorphous Ti(Nb)-Si-C coated alloy exhibits significantly enhanced oxidation resistance, and the oxidation kinetics obey the parabolic law with a low parabolic rate of 9.36 × 10-15 g2·cm-4·s-1. A dual-layer oxide scale is formed composed of an inner layer rich in Cr2O3 and an outer layer rich in rutile TiO2 and amorphous SiO2. MnCr2O4 appears at the interface between the inner and outer oxide layers. Meanwhile, the amorphous Ti(Nb)-Si-C coating also effectively blocks the outward diffusion of Cr. In addition, the coated steel presents good electrical properties with an area-specific resistance (ASR) of 13.57 mΩ·cm2 at 800 °C after oxidation at 800 °C in air for 500 h.

Effects of Mn concentration on durability of Fe-Cr ferritic steels dispersed with La2O3 for solid oxide fuel cell interconnects

This study investigates the effect of Mn concentration in La2O3-dispersed ferritic steel on the oxidation behavior, area-specific resistance (ASR), and chromium volatilization for solid oxide fuel cell (SOFC) interconnects. The results show that increasing Mn concentrations lead to a gradual increase in the oxidation rate of the alloy samples. The alloy with 0.3 wt% Mn primarily forms Cr2O3 during the initial oxidation stage, whereas the alloys containing more than 1.0 wt% Mn promote the growth of (Mn,Cr)3O4, leading to an accelerated oxidation rate. The ASR value exhibits a decreasing trend initially, followed by an increasing trend, as the Mn concentration in the alloy increases from 0.3 to 2.0 wt%. The alloy with 1.0 wt% Mn shows the lowest ASR value of 12.2 mΩ cm2 after 1000 h. The alloys demonstrate a tendency to reduce chromium volatilization as the Mn concentration increases, which can be attributed to the growth of (Mn,Cr)3O4 at the topmost layer of the oxide. The study suggests that controlling the Mn concentration in the Fe-Cr alloys can help in reducing the ASR values and Cr volatilization, making the ferritic steel alloy with 1.0 wt% Mn an effective choice for SOFC interconnects.

Oxidation and electrical properties of nanostructured CeO2–Ni coating on solid oxide fuel cell interconnects

A nanostructured CeO2–Ni coating is prepared on a steel substrate via two-step electrodeposition. An oxide scale with a multi-layer structure of Cr2O3–(Mn,Cr)3O4–(Ni,Mn,Cr)3O4–NiO has developed on the coated steel after 1680 h of oxidation in air at 800 °C. CeO2 is uniformly distributed in the spinel and NiO layers. The synergistic effects originating from nanostructured CeO2 and NiO not only reduce the growth rate of Cr2O3 and suppress the outward diffusion of Cr, but also improve the electrical performance of the surface scale.

Development of Mn-Co and Mn-Co-CeO2 Coatings on Crofer 22 APU Steel for Solid Oxide Fuel Cell Interconnects

The present study compared the performance of uncoated, Mn-Co and Mn-Co-CeO2-coated Crofer 22 APU ferritic stainless steel interconnects. The samples were oxidized for 500 h in an electric furnace at 800 °C. The surface morphology and phase structure of the samples before and after oxidation were examined using FESEM microscopy and X-ray diffraction (XRD) analysis, respectively. The electrical conductivity evaluation of the samples was also conducted by measuring the area specific resistance (ASR). The results indicated that the weight gain of the uncoated, Mn-Co and Mn-Co-CeO2-coated samples after 500 h of oxidation was 0.55, 0.58 and 0.27 mg.cm−2, respectively. Additionally, a comparison of the oxidation kinetics of the experimental samples revealed that the oxidation rate constant of the Mn-Co-CeO2-coated steel is 10 and 40 times lower than that of the Mn-Co-coated and uncoated steel, respectively. XRD analysis of the samples after oxidation confirmed the presence of MnCo2O4, Co3O4, and (Mn,Cr,Co)3O4 in the oxidized Mn-Co-coated, MnCo2O4, Co3O4, and CeO2 in the oxidized Mn-Co-CeO2-coated, and the chromium-containing phases (FeCr2O4, MnCr2O4, and Cr2O3) in the oxidized uncoated samples. The presence of phases with high electrical conductivity in the oxidized coated samples reduced their activation energy for conduction compared to the oxidized uncoated sample.

Supersonic spray derived Cu–Co oxide coating on SUS441 to protect the metallic interconnect and to increase stability of SOFC cathode

The interconnects for solid oxide fuel cells (SOFCs) are usually ferritic stainless steels such as SUS441. These steels have issues of oxidation and Cr diffusion under the operational high temperatures, which can cause serious deterioration of the cathode performance. In order to improve the electrode stability and the oxidation resistivity, this work deposits Cu–Co oxide coating on SUS441 interconnects via room-temperature supersonic spray using fine CuCo2O4 spinel-based oxide powders synthesized via a combustion process. The coating can significantly inhibit the growth of chromium oxide scale on the steel surface and increase the interconnect conductivity by a factor of 4.4 in 200-hr oxidation test in air. The coating effectively prevents Cr diffusion to the cell composed of La0·6Sr0·4FeO3-δ electrode and La0·8Sr0.2Ga0.8Mg0·2O3-δ electrolyte. As a result, the coating substantially increases the cathode performance by reducing the interfacial polarization resistance and improving the stability in 100-hr test at 800 °C. This study demonstrates that this high-performance protective Cu–Co oxide coating has a promising application prospect for SOFC interconnects.

Efficient FeCoNi/CeO2 coatings for solid oxide fuel cell steel interconnect applications

A reactive element layer is introduced into the coating system to enhance the properties of FeCoNi coating for solid oxide fuel cell interconnects. An FeCoNi/CeO2 dual-layer coating is produced on stainless steel by sol-gel and magnetron sputtering, which is subsequently exposed to 800 °C for a duration of up to 20 weeks. The thermal treatment results in the formation of (Fe,Co,Ni)3O4 spinel and (Ni,Co)O, with CeO2 embedded in Cr2O3 that formed beneath the spinel. The FeCoNi/CeO2 coating demonstrates better effectiveness than FeCoNi coating in acting as a barrier against Cr out-diffusion. Moreover, adding CeO2 significantly suppresses the interfacial reaction of Cr2O3 and spinel. An area-specific resistance of 10 mΩ cm2 is obtained for the FeCoNi/CeO2 coated steel at 800 °C after 20 weeks.

Metallic Co with Reactive Element Oxide Composite Coatings for Solid Oxide Fuel Cell Interconnect Applications

Metallic Co with Reactive Element Oxide Composite Coatings for Solid Oxide Fuel Cell Interconnect Applications

In-situ formed CuFe2O4 spinel coating by electroplating method for solid oxide fuel cell interconnect

A protective coating layer on chromium-containing interconnects is critical for preventing chromium diffusion and slowing the increase in area specific resistance (ASR) in solid oxide fuel cells (SOFC). In the present work, a stoichiometric CuFe2 alloy is successfully electroplated on the surface of SUS 430, and a CuFe2O4 spinel coating is formed through in-situ thermal oxidation. Density functional theory calculations and COMSOL simulations are employed to reveal the mechanism of CuFe2 alloy electroplating. The dense CuFe2O4 coating effectively blocks the outward volatile Cr species and suppresses the ASR increase in a long-term test (at 800 °C in the air for 1000 h). Furthermore, the practical application of CuFe2O4-coated SUS 430 is evaluated using a three-electrode cell, showing much improved durability due to the mitigated chromium poisoning. These results demonstrate that the CuFe2O4 coating significantly reduces the out-migration of Cr species from the interconnects. This work provides guidance on electroplating strategy for the future development of alloy coatings for SOFC interconnects.

Experimental Investigation of the Initial Stage of the Oxidation Mechanism of Co Coating for Solid Oxide Fuel Cell Interconnects at 650 °C

The evolution of Co coating in solid oxide fuel cells (SOFCs) under low and medium temperatures (<700 °C) is different from that under high temperatures. In this context, the oxidation corrosion of 441 ferritic stainless steel (FSS) with a Co coating in air under 650 °C was investigated. The results indicated that the Co coating was oxidized rapidly and a Co3O4 spinel layer formed in the initial exposure of 5 min, which improved the oxidation resistance of 441 steel. After oxidation at 650 °C for 120 h, a Cr2O3 layer with a thickness of 0.2–0.4 μm was observed on the surface of bare 441 steel, while the surface oxide scale of Co-coated 441 steel samples was composed of an inner Cr2O3 and Fe2O3 solid solution (0.1–0.3 μm thick), an intermediate (Fe, Co)3O4 layer and an outer Co3O4 spinel layer, respectively. The Co-coated 441 steel sample exhibited better electrical properties. After oxidation at 650 °C for 120 h, the area-specific resistance (ASR) of the Co-coated steel was 3.73 mΩ·cm2, which was 25.4% lower than that of bare 441 steel as 5 mΩ·cm2. Furthermore, the thermal growth process and protection mechanism of Co coating at 650 °C were discussed.

Investigation of CuxMn3-xO4 coatings for solid oxide fuel cell interconnect applications

In order to investigate the performance of Cu–Mn spinel coatings used for solid oxide fuel cell interconnect, a series of CuxMn3-xO4 (x = 0.6, 0.8, 1.0, 1.1, 1.2, and 1.3) coatings are prepared using supersonic spraying via subsequent sintering. The chemical composition and distribution, lattice structures, morphology, and electrical properties are characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and area-specific resistance (ASR) measurements. The experimental results show that with the increase of Cu concentration in the coatings, the crystalline structure gradually changes from inverse spinel into normal spinel, especially when 0.6 ≤ x ≤ 1.0. An improvement in electrical conductivity accompanies the crystalline structural transformation. The coatings show high electrical conductivity when 1.0 ≤ x ≤ 1.3, especially from 600 to 800 °C. However, the scale (mainly Cr2O3) is reactive to the CuxMn3-xO4 coatings (especially when 1.1 ≤ x ≤ 1.3) at elevated temperatures. During the long-term experiment, the reaction corrodes the coatings, consumes the materials in coatings and scale, and forms new phases (mainly (Cu,Mn,Cr)3O4 and (Cr,Mn)3O4). This phenomenon (Cr-corrosion) aggravates the Cr diffusion, causes a quick increase of ASR, and weakens the oxidation resistance. The adverse effects of Cr-corrosion can be alleviated by appropriately lowering the operating temperature. Among the coatings, Cu1.0Mn2.0O4 exhibited good electrical conductivity and soundness against the Cr-corrosion and can be considered a competitive candidate for SOFC applications.

Thermal expansion of Crofer 22 APU steel used for SOFC interconnect using in-situ high temperature X-ray diffraction

Crofer 22 APU is ferritic stainless steel extensively used as metallic interconnect material in Solid Oxide Fuel Cell (SOFC) applications. The interconnects are exposed to both oxidizing and reducing atmospheres at high temperatures. As SOFCs are operated above 700 °C, understanding the thermal expansion behavior of the interconnect material with other components (anode, cathode, electrolyte) of the fuel cells is essential. Metallic interconnects should have a matchable thermal expansion to other ceramic materials such as anode, cathode, and solid electrolyte used in SOFCs. The present study evaluates the thermal expansion of Crofer 22 APU steel from 25 to 950 °C in a controlled atmosphere (10-4 mbar pressure) using in-situ high-temperature X-ray diffraction (XRD). The XRD patterns were analyzed using the ‘High Score Plus Software’ attached to the system, and the phases were identified using the standard Crystallographic Open Database (COD). The coefficient of thermal expansion (CTE) was determined based on the change in lattice parameter/peak shift to a lower 2θ value as a function of temperature. The normal XRD data showed no oxide formation on the Crofer steel after heating until 950 °C in in-situ high-temperature conditions.The peak shift to the lower 2θ degree observed in the XRD data was due to the relaxation of residual stress upon heating. The isothermal section and phase fraction of Crofer 22 APU alloys are analyzed with the help of thermo-calc with the iron database of TCFE7. The Fe-rich bcc phase was found to be stable up to high temperatures. The major phases are the Fe-rich bcc, Cr-rich BCC, and sigma phase in the solid state. The minor phases are FCC, M3P, TiC, Laves, and Ti4C2S2. The calculated lattice parameter of the Fe-rich BCC phase matches with the experimentally calculated data using XRD. The thermal expansion of Crofer 22 APU was found to be 11.9181 × 10-6 /°C at 950 °C. The in-situ high-temperature XRD technique has been an effective methodology for determining the thermal expansion behavior of the as-received Crofer steel.

Oxidation and Scale Adhesion of a Type 430 Stainless Steel in Ar–CO2 Gas Mixtures at 800 °C

Biogas is an alternative source of fuel potentially used to run solid oxide fuel cells (SOFCs). It mainly consists of CH4 and CO2 which can degrade the SOFC interconnect, which is typically made of stainless steel. To investigate the effect of each gas constituent, we focussed here on the effect of CO2 on high-temperature oxidation behavior of and scale adhesion on the stainless steel interconnect, Type 430 stainless steel. The samples studied were oxidised in CO2 at contents of 5–100% at 800 °C. The oxidation kinetics were found to be parabolic with the rate constant increasing when the CO2 content increased. The scale adhesion was assessed using a tensile-test method. The scale formed in the atmosphere containing higher CO2 content exhibited poorer scale adhesion, as indicated by a lower strain initiating the first spallation and a larger spallation percentage after the first spallation took place. The worsened scale adhesion relates to pores formed at the scale/steel interface. The adhesion energies were further quantified giving the values of about 40–100 J m–2. Oxidation mechanisms were suggested based on the dependence of the parabolic rate constant on the oxygen partial pressure and the inward diffusion of carbon-bearing species.